This invention relates to an improved method and apparatus for the sharpening of knives and blades.
There are myriads of knives and the like whose cutting edge must be sharpened either initially or following use. The term "knife" includes professional knives, household knives, blades, swords, surgical tools, razor blades, scissors, chisels, plane blades, and other surfaces having a cutting edge. Commonly household knives and the like are sharpened during manufacture by bringing the cutting edge facets in contact with an abrasive wheel, sometimes in the presence of a coolant such as water or water/oil emulsion particularly where the wheel rotates at high speed. The knife is usually held parallel to and against the perimeter surface (thickness) of the abrasive wheel (FIG. 1) so that moving abrasive elements on the perimeter surface move essentially perpendicular to the long axis of the knife edge. The grit or agglomerate particle size employed in such wheels is commonly such that grooves on the order of 1/4 to 2 mils wide and deep are cut into the knife surface more or less perpendicular to the edge (FIG. 14). These grooves create in effect a serrated edge on the knife that severs largely through a tearing action.
The average commercial knife when viewed with optical magnification can be seen to have an edge somewhat similar to a serrated bread knife. The microteeth on such knives created by the serration become bent during use and commonly are straightened by means of a steel "sharpening" rod that realigns the microteeth. After several "resharpenings" with a steel rod, the teeth become weak and break off, and the knife needs to be reground to be an effective cutting tool. The resharpening process usually consists of again presenting the knife edge to the edge of an abrasive wheel surface.
Household knife sharpeners sold by a variety of manufacturers incorporate high-speed cylindrical stones (FIG. 3) rotating at speeds of about 3000 RPM with surface velocities up to 2000 feet per minute as described in U.S. Pat. No. 2,775,075. The knife cutting edge facet is brought into contact with the beveled edge of a rotating stone so that the abrasive surface is moving in a relatively fixed or limited number of directions relative to the knife edge. These contain coarse grits that grind the knife cutting edge facets, leaving a poorly defined knife edge. At these high abrasive velocities, if the knife is moved nonuniformly or abruptly along the rotating stone, it is possible to create an undesirable scallop on the edge or to overheat the knife edge locally, degrading the temper or gouging the surface of the knife cutting edge facet. Sharpeners of this type are sometimes incorporated as part of household can openers.
An assortment of abrasive rods, sticks, and flat stones are available that are used in a variety of manual sharpening methods. Manual methods lack adequate means to consistently control the sharpening angle and the resulting knife edge is neither well defined nor uniformly sharp.
One manual method of resharpening knives consists of manually stroking the knife cutting edge facet across a static abrasive surface such as Arkansas stone (FIG. 2), carborundum or commercial alumina. Such sharpening stones usually must be coated with oil, or water, during the sharpening process in order to float off sharpening debris removed during sharpening from the knife cutting edge facets and to minimize loading the pores of the stone with abrasive and metallic particles that reduce edge quality and the sharpening rate. Manual methods are seriously disadvantaged by the lack of reproducible motion during individual strokes, by variations in abrading rates during strokes, and by poor angular control. With manual methods it is virtually impossible either to maintain a constant angle of the cutting edge facet relative to the abrasive surface during the manual stroking process, to hold uniform pressure throughout a sharpening stroke, or to avoid damage to the edge from accumulated sharpening debris on the abrasive surface with the consequence that only those highly skilled can hope to obtain a satisfactorily sharp edge.
A major disadvantage of prior art methods is that the edge tends to be left with a sizeable burr, i.e., a curled-over edge of metal on the last unsharpened facet of the blade edge. The presence of a sizeable burr is undesirable as it leaves a poorly deformed, dull, and weak edge on the knife. Both prior art mechanical and manual means leave the knife cutting edge facet scratched along the edge and, in effect, establish a serrated edge that tears while it cuts.
Another type of sharpener, for microtome knives, is described in U.S. Pat. Nos. 3,041,790 and 3,844,067. It utilizes a highly complex arrangement to slowly stroke the knife cutting edge facet in a straight line as it is held against a glass plate coated with loose abrasive material in a suspension. The glass plate is translated laterally and slowly in a circular path for the purpose of keeping the loose abrasive particles more or less evenly dispersed over the plate surface and to reduce their tendency to pile up in small areas on the plates. In these sharpeners the knife is held with pressure against the plate and ground first on one side and then the other by moving the plate or knife slowly and repetitively in essentially long straight lines. The energy of sharpening is provided predominantly by the straight line motion of the knife relative to the loose abrasive on the plate. The result is a micro serrated edge on the knife.
Manufacturers of microtome sharpeners, such as the Thomas Dalton Microtome Knife Sharpener, as described in U.S. Pat. No. 3,874,120 and Bulletin No. 164 of Arthur H. Thomas Company, teach the merits of abrading the knife cutting edge facet to create sets of microscopic scratches aligned at two different angles to the edge and meeting at the edge so as to generate a uniform cross-hatched "X" pattern on the knife facets. This action, like others, tends to create microteeth on the cutting edge with the attendant disadvantages discussed above.
Other known knife sharpening methods include moving water-cooled sandstone wheels or endless abrasive-coated belts. These move the abrasive in a direction essentially perpendicular to the knife edge, thus creating grooves on the facet and microteeth on the edge. Lack of surface planarity of abrasive surface and poor control of the knife position and the angle of the cutting edge facet in these sharpeners commonly leave imperfections along the knife edge. These sharpeners are expensive and often too complex for common household use. Commercially it is commonly necessary to use a fabric buffing wheel to remove burrs remaining after use of such sharpeners.
U.S. Pat. No. 2,645,063 and related U.S. Pat. No. 2,751,721 describes sharpeners that incorporate a magnet. The magnetic field is not incorporated as a part of the knife guide nor to support the weight of the knife. Also its geometry and field orientation renders it ineffective for removal of sharpening debris from the abrasive surface.
Prior art commonly teaches the use of higher surface speed of the abrasive in motor driven sharpening equipment. As described in U.S. Pat. No. 2,775,075 "it has been determined experimentally that the ordinary steel knife cannot be sharpened effectively if the cutting velocity is less than about 500 feet per minute."
Prior art teaches in large that the preferred means to create fine cutting edges is to maintain a motion of the abrasive in a direction largely perpendicular or at some relatively fixed angle relative to the length of knife edge. The result of prior art methods often is a serrated knife edge complete with gouges, edge burrs, and often burned metal. None of these described known means of sharpening have proven wholly satisfactory for sharpening of knives.